Daily mRNA oscillations of circadian clock genes largely depend on transcriptional regulation. However, several lines of evidence highlight the critical role of post-transcriptional regulation in the oscillations of circadian mRNA oscillations. Clearly, variations in the mRNA decay rate lead to changes in the cycling profiles. However, the mechanisms controlling the mRNA stability of clock genes are not fully understood. Here we demonstrate that the turnover rate of mouse Period3 (mPer3) mRNA is dramatically changed in a circadian phase-dependent manner. Furthermore, the circadian regulation of mPer3 mRNA stability requires the cooperative function of 5'- and 3'-untranslated regions (UTRs). Heterogeneous nuclear ribonucleoprotein Q (hnRNP Q) binds to both 5'- and 3'-UTR and triggers enhancement of translation and acceleration of mRNA decay. We propose the phase-dependent translation coupled mRNA decay mediated by hnRNP Q as a new regulatory mechanism of the rhythmically regulated decay of mPer3 mRNA.